A common type of fiber optic network is the Wavelength Division Multiplexed (WDM) network. In a typical WDM network, a multifrequency source is used to generate two or more signals of distinct wavelengths. A single fiber is used to carry the signals to a remote router where they are distributed to network subscribers according to their wavelengths. Similarly, in networks which provide for two-way signaling, the router may be used to combine signals generated by the subscribers.
To maximize the efficiency of a WDM network, the router must accurately track the source wavelengths. The router is essentially a comb filter which, ideally, has its lobes centered on the source signal wavelengths. Accordingly, when the lobes of the router are not aligned with the source wavelengths the router will attenuate the signals intended for the subscribers, the level of attenuation being proportional to the amount of misalignment between the source and the router. Moreover, misalignment of the source and router can result in crosstalk between router outputs, decreasing performance even further. Misalignment may occur, for example, when a temperature change at the source causes the source frequencies to drift or, similarly, when a temperature change at the router causes the router filter bands to drift.
In an attempt to eliminate misalignment between sources and routers network designers have proposed loopback WDMs. In a loopback system, a portion of the signal received by a subscriber is fed back, via a separate feedback fiber, to a central station. The power level of the feedback signal is examined. Any misalignment between the source and router shows up as a drop-off in the power level of the feedback signal and realignment may be achieved by adjusting the source wavelengths to maximize the feedback signal. Such a system, however, requires a separate fiber line to provide the feedback to the central station, and therefore the overall cost of a network employing the system is increased.